The present invention pertains generally to devices for controlling the movement of electrolyte solutions. More particularly, the present invention pertains to electro-osmotic pumps. The present invention is particularly, but not exclusively, useful for an electro-osmotic pump that creates a charged surface with a ferroelectric material.
The electro-osmotic effect can be employed to pump or otherwise control the movement of electrolyte solutions. Devices utilizing the electro-osmotic effect are particularly applicable in micro-fluidics where the manipulation of small amounts of electrolyte solution is required to perform chemical or biochemical reactions. These micro-fluidic processes are often carried out on xe2x80x9cbiochipsxe2x80x9d or xe2x80x9cbioarraysxe2x80x9d and have found increasing usage because only small quantities of reactants and enzymes are needed to conduct each analysis.
To explain the electro-osmotic effect, consider an electric field applied to an electrolyte solution. The electrolyte solution generally contains positive ions, negative ions and a medium for the ions such as water. When the electric field is applied to the electrolyte solution, the positive ions receive a force in the direction of the electric field. Likewise, the negative ions receive a force that is equal in magnitude to the force received by the positive ions, but applied in a direction opposite to the force on the positive ions. The result is that the solution as a whole receives a net force of zero, and the electrolyte solution as a whole does not flow. To summarize, the mere placing of an electrolyte solution in an electric field does not produce an electro-osmotic effect.
Now consider the introduction of a charged surface into the electrolyte solution. Depending on whether the charged surface is acidic (negatively charged) or caustic (positively charged), the charged surface will attract either positive or negative ions from the solution. When an electric field is applied parallel to the charged surface, the resulting electric force acting on the ions that are bound to the charged surface is generally transmitted to the charged surface. On the other hand, the oppositely charged ions (those ions not attracted to the charged surface) are free to move under the influence of the electric field. This electro-osmotic effect causes the solution to receive a net force from the applied electric field, which in turn causes the solution to flow. The direction of flow depends on the polarity of the charged surface, as well as the direction of the applied electric field.
It is known that the electro-osmotic effect can be used to effectuate a simple fluid pump. For example, a pair of electrodes (driving electrodes) can be inserted into the lumen of a tube for contact with an electrolyte solution to create an electric field along the length of the tube. In this arrangement, the inner wall of the tube can be coated with an acidic or caustic material that attracts positive or negative ions from the solution. A voltage source can then be activated to create a potential difference between the electrodes. In response, the solution will flow along the length of the tube. Note that for the simple pump described in this example, the solution will not flow in response to an alternating current (AC) applied to the electrodes, because the time-averaged force on the ions that are not attracted to the tube wall will be zero.
Electrophoresis is often used to separate charged macromolecules (by migration) from a stagnant or non-flowing solution. In these electrophoresis operations, the electro-osmotic effect is undesirable because it causes the solution to flow. To avoid the electro-osmotic effect, the vessel wall can be coated with a passivating material such as Teflon(copyright) which does not interact with either the positive ions or the negative ions.
The present invention recognizes that a ferroelectric material can be used to create the charged surface that is required to produce the electro-osmotic effect. By xe2x80x98temporarilyxe2x80x99 applying an electric field to the ferroelectric material, the ferroelectric material can be xe2x80x98permanentlyxe2x80x99 polarized allowing creation of a charged surface for a device featuring an electro-osmotic effect. Subsequently, the ferroelectric material can be depolarized to create a passivated surface and thereby eliminate any electro-osmotic effect within the device. Depending on the application, the ferroelectric surface can be polarized to produce a surface that either attracts positive ions or negative ions. Further, the magnitude of polarization and thus the total charge placed on the ferroelectric surface can be varied during the operation of the device.
Importantly, as detailed further below, the use of a ferroelectric material allows an electro-osmotic effect to be created when an AC current is applied to the driving electrodes. When an AC current is used, the driving electrodes are not necessarily required to be in direct contact with the electrolyte solution. Rather, a dielectric material can be interposed between the driving electrode and the solution. This is particularly advantageous in situations where direct contact between the electrodes and the solution may be detrimental due to electrochemical reactions at the surface of the electrodes.
Ferroelectric materials differ from ordinary dielectric materials. In an ordinary dielectric material, the electric displacement, D, is generally proportional to the electric field, E. The ratio of the electric displacement and the electric field being the dielectric constant xcex5. Since the relationship between the electric displacement, D, and the electric field, E, is linear, an ordinary dielectric material does not retain an electric displacement after removal of an electric field.
The ferroelectric material is analogous to the more familiar ferromagnetic materials such as ferromagnetic iron except the magnetic field and the magnetic induction are replaced by the electric field, E, and the electric displacement, D. The relationship between the electric field, E and the electric displacement, D, of the ferroelectric material is depicted in FIG. 1. In FIG. 1, point xe2x80x9caxe2x80x9d shows the ferroelectric material in the non-polarized state with E=0 and D=0. When a positive electric field is applied and increased, the relationship between D and E follows the curve from point xe2x80x9caxe2x80x9d to point xe2x80x9cbxe2x80x9d where a maximum displacement DMAX occurs. A subsequent decrease in the electric field, E, causes the displacement, D, to decrease along the curve between points xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d. At point xe2x80x9cc,xe2x80x9d the electric field, E, is zero but the displacement, D, is finite. This is the xe2x80x98poledxe2x80x99 state and the value of D at point xe2x80x9ccxe2x80x9d is known as the remnant polarization. A subsequent reversal of the electric field causes the remnant polarization to vanish (moving along the curve from point xe2x80x9ccxe2x80x9d to point xe2x80x9cdxe2x80x9d in FIG. 1). The relationship between E and D then follows a typical hysteresis curve, passing through points xe2x80x9ce,xe2x80x9d xe2x80x9cfxe2x80x9d and xe2x80x9cgxe2x80x9d as shown in FIG. 1.
The remnant polarization of a ferroelectric material can be removed by a method similar to the depolarization of a magnet. Specifically, when an alternating electric field of decreasing amplitude is applied, the area enclosed by the hysteresis curve becomes smaller and smaller as the amplitude of the alternating electric field is decreased. Eventually, the remnant polarization decreases to zero and the ferroelectric material returns to its original unpolarized state (point xe2x80x9caxe2x80x9d in FIG. 1).
A wide range of the ferroelectric materials are available, including the metal-titanates such as barium-titanate, metal-tantalates, metal-niobates and metal-tungustates. Ferroelectric materials are known that have a maximum displacement of several tenths of Coulomb per square meter. When these ferroelectric materials are polarized, a surface charge of about 10% of available surface lattice sites can result.
When a ferroelectric material is used as the tube material in the simple fluid pump example described above, the electro-osmotic force F that the fluid receives is given by
F=2xcfx80aLDEdxe2x80x83xe2x80x83[1]
where a is the radius of the tube, L is the length of the ferroelectric tube, D is the electric displacement after poling and Ed is the driving electric field. The electro-osmotic force is balanced against the viscous force when the fluid flows at the average velocity of  less than V greater than  in the tube of the length Lxe2x80x2. We obtain
 less than V greater than =[aL/4Lxe2x80x2][DEd/xcexd]xe2x80x83xe2x80x83[2]
where xcexd is the viscosity of the fluid. A set of example parameters may be xcexd=10xe2x88x923 kg mxe2x88x921sxe2x88x921, a=1 mm, [Lxe2x80x2/L]=10, D=0.01 coulomb/mxe2x88x922 and Ed=100 v/m. The velocity is  less than V greater than =2.5 cm/s.
As discussed above, in certain situations it may be desirable to avoid direct contact between the driving electrodes and the electolyte solution. In this case it is possible to interpose a dielectric material between the driving electrodes and the electrolyte solution. When this is done, an AC voltage can be applied to the driving electrodes, and the ferroelectric material can be continuously exposed to an alternating electric field to create an electro-osmotic force.
Referring again to FIG. 1, it can be seen that for a given value of the electric field, E, the displacement, D, of the ferroelectric material is different from that of the dielectric material. Therefore, the surface charge on the ferroelectric material and the dielectric material will have a different magnitude and will not cancel each other. Thus, the fluid will have net free charge and the application of an AC driving electric field will result in an electro-osmotic force. The effective displacement Deff can be defined by
Deff=Dxe2x88x92EDmax/Emaxxe2x80x83xe2x80x83[3]
where Dmax and Emax are defined in FIG. 1. The effective displacement, Deff, (which already accounts for the presence of the dielectric surface) can be used to calculate the electro-osmotic effect.
Although the hysteresis curves generally vary among different ferroelectric materials, the specific hysteresis curve for the actual ferroelectric material used in a device can be used to calculate the effective displacement, Deff. For example, the hysteresis curve shown in FIG. 1 from point xe2x80x9cexe2x80x9d to point xe2x80x9cbxe2x80x9d can be assumed to have the form
D=Dmaxxe2x88x92{4Dmax2xe2x88x92[xcex5*E+Dmax]2}1/2xe2x80x83xe2x80x83[4]
and from point xe2x80x9cbxe2x80x9d to point xe2x80x9ce,xe2x80x9d
D=xe2x88x92Dmax+{4Dmax2+[xcex5*Exe2x88x92Dmax]2}1/2xe2x80x83xe2x80x83[5]
where xcex5*=Dmax/Emax. Accordingly the effective displacements are given by
Deff=Dmaxxe2x88x92xcex5*Exe2x88x92{4Dmax2xe2x88x92[xcex5*E+Dmax]2}1/2xe2x80x83xe2x80x83[6]
and
Deff=Dmaxxe2x88x92xcex5*E+{4Dmax2xe2x88x92[xcex5*Exe2x88x92Dmax]2}1/2xe2x80x83xe2x80x83[7]
If an AC electric field given by
E=Emax cos xcfx89txe2x80x83xe2x80x83[8]
is applied, where xcfx89 is the angular frequency, the effective displacements in each half cycle become
Deff=Dmax[1xe2x88x92cos xcfx89txe2x88x92{4xe2x88x92[1+cos xcfx89t]2}1/2]xe2x80x83xe2x80x83[9]
for xe2x88x92xcfx80xe2x89xa6xcfx89txe2x89xa60
Deff=Dmax[xe2x88x921xe2x88x92cos xcfx89t+{4xe2x88x92[1xe2x88x92cos xcfx89t]2}1/2]xe2x80x83xe2x80x83[10]
for 0xe2x89xa6xcfx89txe2x89xa6xcfx80
The amplitude and the phase of the Fourier component at the frequency xcfx89 is given by
Deff=0.76Dmax cos[xcfx89t+xcex1]xe2x80x83xe2x80x83[11]
with xcex1=73.3xc2x0. The numerical coefficient and the value of the phase angle are associated with the particular hysteresis characteristics that were assumed.
By applying the driving electric field at the same frequency, xcfx89, but with different phase xcex2, namely Ed cos[xcfx89t+xcex2]. The force F on the fluid is given by
F=2xcfx80aLDmaxEd[0.76]cos[xcfx89t+xcex1]cos[xcfx89t+xcex2]xe2x80x83xe2x80x83[12]
and the time averaged force  less than F greater than  becomes
 less than F greater than =2xcfx80aLDmaxEd[0.38]cos[xcex1xe2x88x92xcex2]xe2x80x83xe2x80x83[13]
This relationship indicates that the force is maximum in one direction when xcex2=xcex1 and a maximum in the opposite direction when xcex2=xcex1+xcfx80. Also, this relationship indicates that the magnitude and the direction of the force,  less than F greater than , can be controlled by adjusting the phase xcex2.
The fluid velocity is given by
 less than v greater than =[aL/4Lxe2x80x2][DmaxEd/v][0.38]cos[xcex1xe2x88x92xcex2]xe2x80x83xe2x80x83[14]
Further, the frequency, xcfx89, of the A.C. poling electric field and the driving electric field can be chosen so that the capacitive impedances of the poling and the driving electrodes are comparable to the resistive impedance of the solution.
The electric resistance, R, between the driving electrodes is given by
R=xcex7L/[xcfx80a2]xe2x80x83xe2x80x83[15]
where xcex7 is the resistivity of the solution. The capacitance, C, of the poling electrodes is given by
Cxcx9c[2xcfx80aL/d][Dmax/Emax]xe2x80x83xe2x80x83[16]
where d is the thickness of the dielectric layer. By equating two impedances,
xcfx89xcx9c[CR]xe2x88x921≈[da]/[2L2xcex7Dmax/Emax]xe2x80x83xe2x80x83[17]
Since the dielectric constant of the dielectric material is much smaller than the dielectric constant Dmax/Emax of the ferroelectric material by a factor of a hundred or more, the capacitance of the driving electrode can be made comparable to that of the poling electrode by choosing the thickness of the dielectric film a hundred times smaller than the thickness of the ferroelectric material.
In light of the above it is an object of the present invention to provide devices suitable for applying an electro-osmotic force to a electrolyte solution to pump the solution through a conduit. It is another object of the present invention to provide an electro-osmotic pump which utilizes a ferroelectric material to create a charged surface thereby allowing the surface to be charged and discharged by the application of an electric field. It is yet another object of the present invention to provide an electro-osmotic pump in which the driving electrodes are not in direct contact with the electrolyte solution. It is yet another object of the present invention to provide a miniaturized device that can forward pump, reverse pump or stop an electrolyte solution in a conduit in response to an electrical signal or voltage. A further object of the present invention is to provide a micro-fluidic network of miniaturized fluid switches that can be manipulated like an electronic circuit. Yet another object of the present invention is to provide a ferroelectric electro-osmotic pump which is easy to use, relatively simple to manufacture, and comparatively cost effective.
The present invention is directed to a device and method for controlling the movement of an electrolyte solution. For the present invention, the device includes a conduit having a first end, a second end and a lumen for containing the electrolyte solution. An opening at each end of the conduit allows electrolyte solution to enter and exit the lumen of the conduit. The device further includes a ferroelectric member that is disposed along a portion of the conduit, between the ends of the conduit. The ferroelectric member is formed with a contact surface for interaction with the electrolyte solution. For the present invention, the ferroelectric member is positioned along a portion of the conduit and is oriented to allow the contact surface to interact with the electrolyte solution in the lumen of the conduit.
A polarizing electrode is positioned adjacent to the ferroelectric member to establish an electric field within the ferroelectric member. For the present invention, the polarizing electrode is electrically connected to a voltage source. When activated, the voltage source causes an electric field to be generated in the ferroelectric member that polarizes the ferroelectric member. The polarizing electrode is configured and oriented relative to the ferroelectric member to establish a charge on the contact surface of the ferroelectric member when the ferroelectric member is polarized. The polarity and magnitude of the charge placed on the contact surface will depend on the polarity and magnitude of the voltage supplied to the polarizing electrode.
Driving electrodes are provided to establish a potential difference in the electrolyte solution. The driving electrodes are positioned along the conduit to establish a potential difference across the portion of the conduit containing the ferroelectric member. Specifically, one driving electrode is positioned between the first end of the conduit and the ferroelectric member, and the other driving electrode is positioned between the second end of the conduit and the ferroelectric member.
In one embodiment of the present invention, a first direct current (DC) voltage source is used to polarize the ferroelectric member. Specifically, the first DC voltage source can be used to place a charge on the polarizing electrode, which in turn, can establish an electric field in the ferroelectric member. As discussed above, due to the shape and orientation of the polarizing electrode relative to the ferroelectric member, an electric field can be established that is oriented within the ferroelectric member to create a charge on the contact surface of the ferroelectric member.
In this embodiment of the present invention, the driving electrodes are positioned in direct electrical contact with the electrolyte solution. Further, for this embodiment, a pair of direct current (DC) voltage sources can be used to establish a potential difference between the first driving electrode and the second driving electrode. The potential differential between electrodes creates a potential difference in the electrolyte solution, which in turn, applies a force on the charged ions in the electrolyte solution.
For the operation of this embodiment, the first DC voltage source is connected to the polarizing electrode and activated to polarize the ferroelectric member and thus create a charge on the contact surface. Once the ferroelectric member is polarized, the first voltage source can be deactivated. Ions in the electrolyte solution having a charge polarity that is opposite to the charge polarity of the contact surface will be attracted to the contact surface. Once the ferroelectric member is polarized, the pair of DC voltage sources can be connected to the driving electrodes to establish a potential difference in the electrolyte solution across the portion of the conduit containing the ferroelectric member. Upon establishment of the potential difference in the electrolyte solution, the electrolyte solution will flow along the conduit. The direction of flow can be reversed by reversing the polarity of the driving electrodes or the polarity of the contact surface. The flow can be slowed or stopped by either changing the magnitude of the potential difference applied to the driving electrodes or by de-polarizing the ferroelectric member. For the present invention, the ferroelectric member can be depolarized by immersing the ferroelectric member in an alternating electric field of decreasing amplitude.
In another embodiment of the present invention, a first alternating current (AC) voltage source having an angular frequency, xcfx89, is connected to the polarizing electrode and activated to establish an alternating electric field in the ferroelectric member. Thus, a time-varying charge is placed on the contact surface of the ferroelectric member. In this embodiment, the driving electrodes are not in direct contact with the electrolyte solution. Rather, a layer of dielectric material is interposed between each electrode and the electrolyte solution. Also, for this embodiment, a second AC voltage source is connected to the driving electrodes to establish an alternating potential difference between the two driving electrodes. For the present invention, both the first and second AC voltage sources have the same angular frequency, xcfx89, but may differ in phase angle, xcfx86. Specifically, the phase difference, xcex94xcfx86, between the first and second AC voltage sources can be varied to control the flow rate of electrolyte solution in the conduit as well as the direction of flow. After the geometry of the device is established and the specific dielectric and ferroelectric materials have been selected, the relationship between flow rate and phase difference, xcex94xcfx86, can be calculated. In accordance with the present invention, the phase difference, xcex94xcfx86, can be varied to maximize the flow rate, reverse the flow direction or stop the flow of electrolyte solution in the conduit.